Like many issues in equine nutrition, there are strong opinions both pro and con regarding alfalfa. There is also much misinformation.
One thing everyone can agree upon is that horses love alfalfa. However, horses also love grain and left to their own devices would consume enough to cause laminitis, severe colic and potentially death. We can’t take their preference as something that should dictate what is fed with no limitations.
Some claim the high protein in alfalfa causes developmental bone/joint diseases in young horses. This is not true. Protein deficiency is far more likely than excess to cause problems in growing horses. A much more probable connection is the very high calcium content of alfalfa which throws the Ca:P ratio way out of the ideal balance.
The high calcium to phosphorus ratio can also cause problems for endurance horses. High calcium diets can cause hormones needed to mobilize calcium ions quickly in times of need, like exercise, to become sluggish. This can contribute to metabolic problems in endurance horses. As a result, riders limit or eliminate alfalfa from the daily diet but do feed it during competitions.
Alfalfa has also been implicated in enterolith formation in horses. Enteroliths are stones in the intestinal tract composed of primarily magnesium, ammonia and phosphate. The high magnesium and ammonia (from protein metabolism) can come from alfalfa. However, this is primarily an issue in California and with Arabian horses. There are many horses in the West/Midwest on full alfalfa diets that do not have this issue. Something else is going on beyond the alfalfa but it remains true that horses, especially Arabians, on alfalfa only diets with no grain are at risk for enteroliths.
It has been said the high protein in alfalfa can strain or damage the kidneys. This is not true. In fact, young growths of pasture have even higher protein levels than alfalfa. The kidneys do process the breakdown products of excess protein but they have no trouble doing this. The only adverse effect is a higher output of urine and obvious ammonia smell in the stall.
Alfalfa is said to be higher calorie than grass hays. It is true that a very high quality dairy alfalfa, with soft stems and few blossoms, may be as much as 20% higher in calories but a very early cutting of grass hay will be a lot closer.
Alfalfa also has a reputation for making horses “hot” in a behavioral sense. This is far from true for every horse but some horses do indeed seem supercharged when fed alfalfa. The reason is unknown.
The main drawback to alfalfa is the very high calcium level. The high calcium:phosphorus ratio is potentially harmful to growing horses and endurance horses. Since horses have high calcium absorption the excess is excreted in urine and contributes to urinary tract sludge or even stones.
As a rule of thumb it is best to limit alfalfa to 10 to 20% of the diet. Alfalfa tea, made by steeping it with boiling water or in a large jar left on the windowsill in full sun, is also a useful addition as a taste tempter for horses with picky appetites.
Eleanor Kellon, VMD